1. Field of the Invention
This invention relates to device processing and, in particular, to semiconductor processing.
2. Art Background
The selective etching of materials in processes such as semiconductor device processing is often required. For example, it is desirable in certain situations, such as in the production of appropriately configured gate oxides, to remove a region of silicon essentially without causing significant damage to an underlying or adjacent region of a silicon oxide, e.g., silicon dioxide. Processes such as plasma etching and reactive ion etching are often utilized to accomplish these results. In these techniques a gas is typically introduced in proximity to the body to be etched and a plasma is established in the gaseous medium producing molecular fragments, atoms, and ions. The resulting energetic entities produced by the plasma are directed towards the substrate and through various mechanisms remove the impacted material. By a particular choice of processing conditions and gases the rate of removal of a given material is, to an extent, controlled relative to the removal rate of other materials.
While in many situations etching involving energetic entities is advantageously employed, it is not without associated difficulties. The energetic particles produced in the plasma often affect even materials which are not etched at a substantial rate by, for example, inducing pitting or by unacceptably modifying surface electronic states. Pitting and electronic state modification for many device applications are not desirable since they often lead to defective device structures and thus device failure. Additionally, the use of a plasma also often leads to the deposition of contaminating materials onto the substrate surface. These contaminating materials, such as compounds produced from the plasma gas, and/or non-volatile metals from the reaction vessel, e.g., aluminum, either degrade device properties or hinder subsequent processing procedures.
Wet etching, i.e., use of a liquid based chemical that rapidly reacts with the material to be etched as compared to its rate of reaction with underlying or masked adjacent materials, is an alternative to plasma etching. Again, although wet chemical etching is advantageously employed in many procedures, it too is not without difficulties. For example, wet chemical etching leads to difficulties in handling and disposal of the associated chemicals. Additionally, if the temperature and concentration of the reagents are not carefully maintained, inconsistent results are obtained. Inconsistent results also arise from the wet etchant failing to reach micron features due to surface tension effects.
Although in many circumstances etching techniques, e.g., plasma etching and wet chemical etching, exhibit a degree of selectivity between the material being etched and adjoining material, this level of selectivity is generally not particularly high. For typical etching systems, selectivity, i.e., the rate of etching of the desired region relative to underlying or unmasked adjacent regions of different compositions, is not greater than 20 to 1. However, there are many applications, for example, the selective removal of silicon filaments from extremely thin silicon dioxide gates in high-speed field effect transistors, that require selectivity which is greater than 100 to 1.
Xenon difluoride in the absence of a plasma has been shown to produce selective etching between silicon dioxide and silicon. (See H. F. Winters et al, Applied Physics Letters, 34, 70 (1979).) However, rare gas halides are generally unstable and relatively costly. As a result, the use of rare gas halides is not a particularly satisfactory solution to the problems associated with other procedures. Therefore, common etching procedures have shortcomings and are not sufficiently selective for some important applications.